Intervertebral allograft spacer

ABSTRACT

An allogenic intervertebral implant for fusing vertebrae is disclosed. The implant is a piece of allogenic bone conforming in size and shape with a portion of an end plate of a vertebra. The implant has a wedge-shaped profile to restore disc height and the natural curvature of the spine. The top and bottom surfaces of the implant have a plurality of teeth to resist expulsion and provide initial stability. The implant according to the present invention provides initial stability need for fusion without stress shielding.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/828,625, filed Apr. 9, 2001, which is a continuation of U.S. patentapplication Ser. No. 09/363,844, filed Jul. 30, 1999 (now U.S. Pat. No.6,258,125, and reissued as U.S. Reissue Patent No. RE38,614), which is acontinuation-in-part of U.S. patent application Ser. No. 09/219,439,filed Dec. 23, 1998 (now U.S. Pat. No. 6,143,033), which claims priorityto U.S. Provisional Application No. 60/095,415, filed Aug. 5, 1998, U.S.Provisional Application No. 60/095,209, filed Aug. 3, 1998, and U.S.Provisional Application No. 60/073,271, filed Jan. 30, 1998, thecontents of all of which are expressly incorporated by reference herein.

FIELD OF THE INVENTION

The present invention is directed to an allogenic implant and, moreparticularly, to an allogenic intervertebral implant.

BACKGROUND OF THE INVENTION

A number of medical conditions such as compression of spinal cord nerveroots, degenerative disc disease, and spondylolisthesis can cause severelow back pain. Intervertebral fusion is a surgical method of alleviatinglow back pain. In posterior lumbar interbody fusion (“PLIF”), twoadjacent vertebral bodies are fused together by removing the affecteddisc and inserting an implant that would allow for bone to grow betweenthe two vertebral bodies to bridge the gap left by the disc removal.

A number of different implants and implant materials have been used inPLIF with varying success. Current implants used for PLIF includethreaded titanium cages and allografts. Threaded titanium cages sufferfrom the disadvantage of requiring drilling and tapping of the vertebralend plates for insertion. In addition, the incidence of subsidence inlong term use is not known. Due to MRI incompatibility of titanium,determining fusion is problematic. Finally, restoration of lordosis,i.e., the natural curvature of the lumbar spine is very difficult when acylindrical titanium cage is used.

Allografts are sections of bone taken from a long bone of a donor. Across section of the bone is taken and processed using known techniquesto preserve the allograft until implantation and reduce the risk of anadverse immunological response when implanted. For example, U.S. Pat.No. 4,678,470 discloses a method for processing a bone grafting materialwhich uses glutaraldehyde tanning to produce a non-antigenic,biocompatible material. Allografts have mechanical properties which aresimilar to the mechanical properties of vertebrae even after processing.This prevents stress shielding that occurs with metallic implants. Theyare also MRI compatible so that fusion can be more accuratelyascertained and promote the formation of bone, i.e., osteoconductive.Although the osteoconductive nature of the allograft provides abiological interlocking between the allograft and the vertebrae for longterm mechanical strength, initial and short term mechanical strength ofthe interface between the allograft and the vertebrae are lacking asevidenced by the possibility of the allograft being expelled afterimplantation.

Currently commercially available allografts are simply sections of bonenot specifically designed for use in PLIF. As a result, the fusion ofthe vertebral bodies does not occur in optimal anatomical position. Asurgeon may do some minimal intraoperative shaping and sizing tocustomize the allograft for the patient's spinal anatomy. However,significant shaping and sizing of the allograft is not possible due tothe nature of the allograft. Even if extensive shaping and sizing werepossible, a surgeon's ability to manually shape and size the allograftto the desired dimensions is severely limited.

Most PLIF implants, whether threaded cages or allograft, are availablein different sizes and have widths that vary with the implant height.For example, the width of a cylindrical cages will be substantiallyequivalent to the height. Although larger heights may be clinicallyindicated, wider implants are generally not desirable since increasedwidth requires removal of more of the facet, which can lead to decreasesstability, and more retraction of nerve roots, which can lead totemporary or permanent nerve damage.

As the discussion above illustrates, there is a need for an improvedimplant for fusing vertebrae.

SUMMARY OF THE INVENTION

The present invention relates to an allogenic intervertebral implant foruse when surgical fusion of vertebral bodies is indicated. The implantcomprises a piece of allogenic bone conforming in size and shape with aportion of an end plates of the vertebrae and has a wedge-shaped profilewith a plurality of teeth located on top and bottom surfaces. The topand bottom surfaces can be flat planar surfaces or curved surfaces tomimic the topography of the end plates. The implant has a channel on atleast one side for receiving a surgical tool. This channel runs in theanterior direction to accommodate a variety of surgical approaches. Athreaded hole on the anterior, posterior, posterior-lateral, or lateralside can be provided for receiving a threaded arm of an insertion tool.

In another embodiment, the implant has an interior space for receivingan osteoconductive material to promote the formation of new bone.

In another embodiment, the implant is made of a plurality ofinterconnecting sections with mating sections. Preferably, the implantis made in two halves: a top portion having a top connecting surface anda bottom portion having a bottom connecting surface. The top connectingsurface mates with the bottom connecting surface when the top and bottomportions are joined. The top and bottom portions have holes that alignfor receiving a pin to secure the top and bottom portions together. Thepin can be made of allogenic bone.

In a different embodiment, the medial side of the implant has ascalloped edge such that when a first implant is implanted with a secondimplant with the medial sides facing each other, the scalloped edgesdefine a cylindrical space.

The present invention also relates to a discrete spacer used inconjunction with any of the other embodiments of the implant. The spacercomprises a piece of allogenic bone conforming in size and shape with aportion of an end plates of the vertebrae and has a wedge-shaped profilewith substantially smooth top and bottom surfaces. The intersectingregions between the top and bottom surfaces and at least one of thelateral sides and the intersecting regions between the anterior andposterior sides and the same lateral side are curved surfaces tofacilitate implantation of the spacer. Thus, the spacer can be implantedthrough an opening on one side of the spinal canal and moved with asurgical instrument to the contralateral side.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a first embodiment of the implant according tothe present invention;

FIG. 2 is a side view of the implant of FIG. 1;

FIG. 3 is a back view of the implant of FIG. 1;

FIG. 4 is a top view of a second embodiment of the implant;

FIG. 5 is a side view of the implant of FIG. 4;

FIG. 6 is a top view of a third embodiment of the implant;

FIG. 7 is a side view of the implant of FIG. 6;

FIG. 8A is a top view of a top connecting surface of a top portion ofthe implant of FIG. 6;

FIG. 8B is a top view of a bottom connecting surface of a bottom portionof the implant of FIG. 6;

FIG. 9 is a perspective view of a fourth embodiment of the implant;

FIG. 10A is a side view of one embodiment of the teeth on the implant;

FIG. 10B is a side view of a second embodiment of the teeth of theimplant;

FIG. 11 is a side view of an embodiment of the implant similar to theembodiment of FIGS. 6-8;

FIG. 12 is a top view of a vertebral bone characteristic of those of thecervical, thoracic, and lumbar spine;

FIG. 13 is a side view of sequentially aligned vertebral bones, such asare found in the cervical, thoracic, or lumbar spine;

FIG. 14 is a posterior view of a sequence of vertebrae; and

FIG. 15 is an end view of another embodiment of the implant.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a top view of a first embodiment of intervertebralallograft spacer or implant 10 according to the present invention.Implant 10 conforms in size and shape with a portion of end plates ofthe vertebrae between which implant 10 is to be implanted. Becauseimplant 10 is an allograft, implant 10 promotes the formation of newbone to fuse the two vertebral bodies together. Although implant 10 willprobably be predominantly used in the lumbar region of the spine,implant 10 can be configured for implantation in any region of thespine. Implant 10 has a plurality of teeth 12 on superior and inferiorsurfaces 14, 16 which provide a mechanical interlock between implant 10and the end plates. Teeth 12 provide the mechanical interlock bypenetrating the end plates. The initial mechanical stability afforded byteeth 12 minimizes the risk of post-operative expulsion of implant 10.Teeth 12 can be pyramid-shaped (FIG. 10A). Preferably, the angle formedfrom the tip to the base is approximately 60°. Alternatively, teeth 12have a saw tooth shape with the saw tooth running in theanterior-posterior direction (FIG. 10B).

As shown in FIG. 2 and FIG. 3, a first lateral side 18 has a channel 20and a second lateral side 22 also has a channel 20. Channels 20 aresized to receive a surgical instrument such as an inserter forimplantation of implant 10. If the inserter has a threaded arm, implant10 can be provided with a threaded hole 24. In FIG. 2, channel 20 isshown extended only partially along first lateral side 18. Channel 20can extend along the entire length of first lateral side 18 as shown inthe embodiment of FIG. 5. In FIG. 3, channels 20 are shown on both firstand second lateral sides 18, 22. It should be noted that implant 10 canalso have no channels or channels on one lateral side only as shown inthe embodiment of FIG. 9.

The dimensions of implant 10 can be varied to accommodate a patient'sanatomy. Typically, implant 10 would have a width between 6-15 mm (inthe medial-lateral direction), a length between 15-30 mm (in theanterior-posterior direction), and a height between 4-30 mm (maximumheight in the superior-inferior direction). The size of implant 10allows implant 10 to be implanted using conventional open surgicalprocedures or minimally invasive procedures, such as laparoscopicsurgery. Additionally, because the width is kept to a restricted sizerange and does not necessarily increase with implant height, tallerimplants can be used without requiring wider implants. Thus, facetremoval and retraction of nerve roots can remain minimal.

In order to restore the natural curvature of the spine after theaffected disc has been removed, implant 10 has a wedge-shaped profile.As shown in FIG. 2, this wedge shape results from a gradual decrease inheight from an anterior side 26 to a posterior side 28. In anatomicalterms, the natural curvature of the lumbar spine is referred to aslordosis. When implant 10 is to be used in the lumbar region, the angleformed by the wedge should be approximately between 4.20 and 150 so thatthe wedge shape is a lordotic shape which mimics the anatomy of thelumbar spine.

In order to facilitate insertion of implant 10, anterior side 26transitions to superior and inferior surfaces 14, 16 with rounded edges30. Rounded edges 30 enable implant 10 to slide between the end plateswhile minimizing the necessary distraction of the end plates.

Although implant 10 is typically a solid piece of allogenic bone,implant 10 can be provided with a hollow interior to form an interiorspace. This interior space can be filled with bone chips or any otherosteoconductive material to further promote the formation of new bone.

FIG. 4 shows a top view of a second embodiment of an implant 40according to the present invention. In general, most of the structure ofimplant 40 is like or comparable to the structure of implant 10.Accordingly, discussion of the like components is not believednecessary. The superior and inferior surfaces 14, 16 of implant 10 areflat planar surfaces. As seen best in FIG. 5, superior and inferiorsurfaces 14, 16 of implant 40 are curved surfaces which still retain thewedge-shaped profile. The curved surfaces of superior and inferiorsurfaces 14, 16 of implant 40 are a mirror-image of the topography ofthe vertebral end plates. Thus, the curved surfaces conform to thecontours of the end plates.

FIG. 6 shows a top view of a third embodiment of an implant 50 accordingto the present invention. In general, most of the structure of implant50 is like or comparable to the structure of implants 10, 40.Accordingly, discussion of the like components is not believednecessary. As best seen in FIG. 7, implant 50 comprises a top portion 52joined to a bottom portion 54. As it may be difficult to obtain a singlesection of allogenic bone from which implant 50 is to be made,fabricating implant 50 in two pieces, i.e. top and bottom portions 52,54, allows smaller sections of allogenic bone to be used. A topconnecting surface 56 and a bottom connecting surface 58 define theinterface between top and bottom portions 52, 54. As shown in FIGS. 8Aand 8B, top and bottom surfaces 56, 58 have ridges 60 that mate withgrooves 62 to interlock top and bottom portions 52, 54. Preferably,ridges 60 and grooves 62 are formed by milling top and bottom surfaces56, 58 in a first direction and then milling a second time with top andbottom surfaces 56, 58 oriented 90° with respect to the first direction.

A pin 64 passing through aligned holes 66 in top and bottom portions 52,54 serves to retain top and bottom portions 52, 54 together. Althoughpin 64 can be made of any biocompatible material, pin 64 is preferablymade of allogenic bone. The number and orientation of pins 64 can bevaried.

FIG. 11 shows an embodiment of an implant 80 which, like implant 50, ismade in multiple pieces. In general, most of the structure of implant 80is like or comparable to the structure of implants 10, 40, 50.Accordingly, discussion of the like components is not believednecessary. Implant 80 has a top portion 82, a middle portion 84, and abottom portion 86. As was the case for implant 80, the surfaces betweenthe portions are mating surfaces with interlocking surface features,such as ridges and grooves. One or more pins preferably hold top,middle, and bottom portions 82, 84, 86 together.

FIG. 9 shows a perspective view of a fourth embodiment of a firstimplant 70 according to the present invention. A second implant 70′,which is substantially similar to first implant 70, is also shown. Ingeneral, most of the structure of first and second implants 70, 70′ islike or comparable to the structure of implants 10, 40, 50. Accordingly,discussion of the like components is not believed necessary. Firstlateral sides 18 of first and second implants 70, 70′ are scalloped tohave a C-shape. When first and second implants 70, 70′ are placed sideby side with the first lateral sides 18 facing each other, a cylindricalspace 72 is formed. When first and second implants 70, 70′ are implantedtogether, cylindrical space 72 can be filled with osteoconductivematerial to help promote the formation of new bone. First and secondimplants 70, 70′ can be provided with locking pins 74 which engageapertures 76 to maintain the spatial relationship between first andsecond implants 70, 70′.

The use of the implant according to the present invention will now bedescribed with reference to FIGS. 12-14 and using posterior lumbarinterbody fusion as an example. As the implant according to the presentinvention conforms in size and shape to a portion of end plates 100,preoperative planning is recommended for proper sizing. Determine theappropriate implant height by measuring adjacent intervertebral discs102 on a lateral radiograph. The implant must be seated firmly with atight fit between end plates 100 when the segment is fully distracted.The tallest possible implant should be used to maximize segmentalstability. Due to variability in degrees of magnification fromradiographs, the measurements are only an estimate.

With the patient in a prone position on a lumbar frame, radiographicequipment can assist in confirming the precise intraoperative positionof the implant. The surgeon incises and dissects the skin from themidline laterally and locates spinous process 104, lamina 106, dura 108,and nerve roots of the appropriate level(s). As much as facets 110 aspossible should be preserved to provide stability to the intervertebralsegment. The surgeon performs a laminotomy to the medial aspect of facet110 and reflects dura 108 to expose an approximately 13 mm window to thedisc space. Disc 102 is removed through the window until only anterior112 and lateral 114 annulus remain. The superficial layers of the entirecartilaginous end plates 100 are also removed to expose bleeding bone.Excessive removal of the subchondral bone may weaken the anteriorcolumn. Furthermore, if the entire end plate is removed, this may resultin subsidence and a loss of segmental stability.

Distraction can be done with either a surgical distractor or a trialspacer implant. In the first method, the distractor blades are placedinto the disc space lateral to dura 108. The curve on the neck of thedistractor should be oriented toward the midline. The distractor bladesshould be completely inserted into the disc space so that the ridges atthe end of the blades rest on vertebral body 116. Fluoroscopy can assistin confirming that the distractor blades are parallel to end plates 100.Correct placement will angle the handles of the distractor cranially.particularly at L5-S1. The handle of the distractor is squeezed todistract the innerspace. The distraction is secured by tightening thespeed nut on the handle.

Using the preoperatively determined size, a trial spacer is inserted inthe contralateral disc space with gentle impaction. Fluoroscopy andtactile judgement can assist in confirming the fit of the trial spaceruntil a secure fit is achieved. Using either the slots or threader holeon the implant, the selected implant is inserted in the contralateraldisc space. Alternatively, the channels on the implant allow distractionand insertion to occur on the same side. Regardless of the side theimplant is inserted in, autogenous cancellous bone or a bone substituteshould be placed in the anterior and medial aspect of the vertebral discspace prior to placement of the second implant. The distractor isremoved and a second implant of the same height as the first implant isinserted into the space, using gentle impaction as before. Preferably,the implants are recessed 2-4 mm beyond the posterior rim of thevertebral body.

As previously noted, the implant according to the present invention canbe inserted using minimally invasive procedures. In some of theseprocedures, only one side of the spinal cord needs to be approached.This minimizes muscle stripping, scar tissue in the canal, and nerveroot retraction and handling. In clinical situations in which bilateralimplant placement is required, proper implantation on the side oppositethe incision can be difficult. FIG. 15 shows a beveled spacer 120 thatfacilitates placement on the side contralateral to the incision. Ingeneral and unless otherwise described, most of the structure of beveledspacer 120 is like or comparable to the structure of implants 10, 40,50, and 80. Accordingly, discussion of the like components is notbelieved necessary. First lateral side 18 transitions to superior andinferior surfaces 14, 16 with rounded edges 30. First lateral side 18also transitions to anterior and posterior sides 26, 28 with roundededges 30. Additionally, spacer 120 has no teeth. The lack of teeth androunded edges 30 enable spacer 120 to slide between the end plate andacross the evacuated disc space (from one lateral annulus to the other)to the contralateral side. As first lateral side 18 is the side thatmust promote movement of spacer 120, the use of rounded edges 30 onsecond lateral side 22 is optionally. Once spacer 120 has been placed onthe side contralateral to the single incision using a surgicalinstrument to push spacer 120, bone graft or other osteoconductivematerial is packed in the disc space. Finally, an implant (any ofimplant 10, 40, 50, 70, or 70′ can be used) is implanted in the sideproximal to the incision.

While it is apparent that the illustrative embodiments of the inventionherein disclosed fulfill the objectives stated above, it will beappreciated that numerous modifications and other embodiments may bedevised by those skilled in the art. Therefore, it will be understoodthat the appended claims are intended to cover all such modificationsand embodiments which come within the spirit and scope of the presentinvention.

1. An intervertebral implant comprising at least one piece of allogenicbone provided with a hollow interior space, the implant having top andbottom surfaces configured and adapted in use to face endplates ofadjacent vertebrae; wherein at least a portion of the top and bottomsurfaces is textured to resist migration of the implant, the implantfurther comprising a mid-plane, and wherein the top surface is inclinedin a range between about 4.2° and about 15° with respect to themid-plane; and wherein the implant has a length between about 8 mm andabout 30 mm.
 2. The implant of claim 1, wherein the length is betweenabout 8 mm and about 18 mm.
 3. The implant of claim 1, wherein thelength is between about 22 mm and about 30 mm.
 4. The implant of claim1, wherein the length is between about 15 mm and about 30 mm.
 5. Theimplant of claim 1, wherein at least one textured portion comprises aplurality of teeth.
 6. The implant of claim 5, wherein the teeth arearranged in a plurality of adjacent rows.
 7. The implant of claim 5,wherein the teeth have a pyramidal shape profile.
 8. An intervertebralimplant comprising at least one piece of allogenic bone provided with ahollow interior space, the implant having top and bottom surfacesconfigured and adapted in use to face endplates of adjacent vertebrae;wherein at least a portion of the top and bottom surfaces is textured toresist migration of the implant, the implant further comprising amid-plane, and wherein the top surface and bottom surface aresubstantially parallel to the mid-plane; and wherein the implant has alength between about 8 mm and about 30 mm.
 9. The implant of claim 8,wherein the length is between about 8 mm and about 18 mm.
 10. Theimplant of claim 8, wherein the length is between about 22 mm and about30 mm.
 11. The implant of claim 8, wherein the length is between about15 mm and about 30 mm.
 12. The implant of claim 8, wherein at least onetextured portion comprises a plurality of teeth.
 13. The implant ofclaim 12, wherein the teeth are arranged in a plurality of adjacentrows.
 14. The implant of claim 12, wherein the teeth have a pyramidalshape profile.
 15. An intervertebral implant comprising at least onepiece of allogenic bone provided with a hollow interior space, theimplant having top and bottom surfaces configured and adapted in use toface endplates of adjacent vertebrae; wherein at least a portion of thetop and bottom surfaces is textured to resist migration of the implant,the implant further comprising a mid-plane, and wherein the top surfaceis inclined in a range between about 4.2° and about 15° with respect tothe mid-plane; the implant further comprising an anterior side and aposterior side; wherein the implant has a height between about 4 mm andabout 100 mm.
 16. The implant of claim 15, wherein the height is betweenabout 10 mm and about 100 mm.
 17. The implant of claim 15, wherein theheight is between about 4 mm and about 30 mm.
 18. The implant of claim15, wherein the height is between about 4 mm and about 20 mm.
 19. Theimplant of claim 15, wherein the height of the anterior side is largerthan the height of the posterior side.
 20. The implant of claim 15,wherein the height of the anterior side is substantially equal to theheight of the posterior side.
 21. The implant of claim 15, wherein atleast one textured portion comprises a plurality of teeth.
 22. Theimplant of claim 21, wherein the teeth are arranged in a plurality ofadjacent rows.
 23. The implant of claim 21, wherein the teeth have apyramidal shape profile.
 24. An intervertebral implant comprising atleast one piece of allogenic bone provided with a hollow interior space,the implant having top and bottom surfaces configured and adapted in useto face endplates of adjacent vertebrae; wherein at least a portion ofthe top and bottom surfaces is textured to resist migration of theimplant, the implant further comprising a mid-plane, and wherein the topsurface and bottom surface are substantially parallel to the mid-plane;the implant further comprising an anterior side and a posterior side;wherein the implant has a height between about 4 mm and about 100 mm.25. The implant of claim 24, wherein the height is between about 10 mmand about 100 mm.
 26. The implant of claim 24, wherein the height isbetween about 4 mm and about 30 mm.
 27. The implant of claim 24, whereinthe height is between about 4 mm and about 20 mm.
 28. The implant ofclaim 24, wherein the height of the anterior side is larger than theheight of the posterior side.
 29. The implant of claim 24, wherein theheight of the anterior side is substantially equal to the height of theposterior side.
 30. The implant of claim 24, wherein at least onetextured portion comprises a plurality of teeth.
 31. The implant ofclaim 30, wherein the teeth are arranged in a plurality of adjacentrows.
 32. The implant of claim 30, wherein the teeth have a pyramidalshape profile.
 33. An intervertebral implant comprising at least onepiece of allogenic bone provided with a hollow interior space, theimplant having top and bottom surfaces configured and adapted in use toface endplates of adjacent vertebrae; wherein at least a portion of thetop and bottom surfaces is textured to resist migration of the implant,the implant further comprising a mid-plane, and wherein the top surfaceis inclined in a range between about 4.2° and about 15° with respect tothe mid-plane; the implant further comprising an anterior side and aposterior side; wherein the implant has a width between about 6 mm andabout 15 mm.
 34. An intervertebral implant comprising at least one pieceof allogenic bone provided with a hollow interior space, the implanthaving top and bottom surfaces configured and adapted in use to faceendplates of adjacent vertebrae; wherein at least a portion of the topand bottom surfaces is textured to resist migration of the implant, theimplant further comprising a mid-plane, and wherein the top surface andbottom surface are substantially parallel to the mid-plane; the implantfurther comprising an anterior side and a posterior side; wherein theimplant has a width between about 6 mm and about 15 mm.